This invention relates to an improved process for the production of cellular polyurethane elastomers from polyisocyanates, relatively high molecular weight polyhydroxyl compounds and water as chain lengthening and blowing agent. The new process makes it possible for cellular polyurethane elastomers to be produced which have a higher density and higher heat distortion temperature. The process also avoids the excessive evolution of carbon dioxide which occurs in the conventional processes and the consequent high pressure produced in the molds.
Among the polyurethane foams, cellular polyurethane elastomers are distinguished by their substantially higher densities (about 450 to 800 kg/m.sup.3) and their improved physical properties. This improves the possibilities of commercial application. Cellular polyurethanes are used, for example, for shoe soles or as buffer elements in mechanical engineering, specifically in the construction of vehicles. They are also becoming increasingly important because of their good thermal stability. The conventional technique for the production of cellular polyurethane elastomers is known from the technical literature, e.g. Kunststoff Handbuch, Volume VII (Polyurethanes), published by Vieweg and Hochtlen, pages 275 to 287). In these processes, relatively high molecular weight polyhydroxyl compounds such as polyesters or polyethers are reacted with diisocyanates to produce isocyanate prepolymers. These prepolymers are then processed into cellular materials by reaction with water to which emulsifiers and, optionally, activators have been added. The water acts as both a chain lengthening agent and a blowing agent.
In order to obtain elastomers with adequate mechanical properties by this prior art process, it is necessary to use a quantity of water as chain lengthening agent which will not result in a cellular elastomer of the desired density (about 450 to 800 kg/m.sup.3). The chain lengthening reaction is accompanied by the production of large quantities of excess carbon dioxide so that it is difficult to produce molded articles having the high densities mentioned above. In Kunststoff Handbuch, Volume VII, page 276, it is proposed to overcome this difficulty by using as cross-linking agent, mixtures of water and a conventional glycol chain lengthening agent. Although higher densities can be obtained at low foaming pressures by means of such formulations, the products are inferior in their dynamic characteristics to those foams which have been cross-linked with water alone. Consequently, they have acquired little commercial importance. When cellular polyurethanes which have been produced with such cross-linking agents (mixtures of glycol and water) are used for shoe soles, their thermal stability is insufficient to withstand the heat produced when a fast running athlete stops dead in his tracks. The reason for this is that cellular polyurethane elastomers of this type begin to soften at 65.degree. C. and sometimes soften completely at 120.degree. C. (thermomechanical analysis and torsion vibration test).
If, on the other hand, a diamine chain lengthening agent is substituted for that portion of water which is not required as blowing agent, the cross-linking reaction in the production of cellular polyurethane elastomers proceeds much too rapidly. This causes processing problems which are virtually insurmountable. Also, the products obtained have a very irregular cell structure.
It was, therefore, an object of the present invention to provide a process by which cellular polyurethane elastomers having a relatively high density could be produced by modifying the usual formulations so that excessive gas pressures would not be produced in the foaming process. In addition, the problem of high heat distortion temperature is avoided.
It has now surprisingly been found that these objects can be achieved by adding very small quantities of aromatic diamines in the production of the cellular elastomers (in principle, at any stage of production). The diamines are present in substantially smaller molar quantities than the water and, therefore, make only a minor contribution to the chain lengthening reaction. Considerable improvements in the properties are nevertheless achieved.